Have people start an empty guild. When you say start, they have to find the best way to convince ppl to join their guild. Some rules can be: no spamming, bribing, etc. They have to have the most ppl in their guild (has to be a sustainable guild!) at the end of a time period (say for ex. 3 days) to win. Newly recruited ppl must not know of the contest (if they do, they'll help one person out for sure.) The gm cannot promote anybody. The participants need to use their own unique strategy. They tell u they're strategy for recruiting by pm'ing you. You can have a raffle for whoever can pick their strategy first.

Sort of confusing.. just a random idea that came out of my mind.